Exposure apparatuses are commonly used to transfer images from a reticle onto a semiconductor wafer during semiconductor processing. A typical exposure apparatus includes an illumination source, a reticle stage assembly that positions a reticle, an optical assembly having an optical axis, a wafer stage assembly that positions a semiconductor wafer, a measurement (or position) system, and a control system. The measurement system constantly monitors the position of the reticle and the wafer, and the control system controls each stage assembly to constantly adjust the position of the reticle and the wafer. The features of the images transferred from the reticle onto the wafer are extremely small. Accordingly, the precise positioning of the wafer and the reticle is critical to the manufacturing of high quality wafers.
There is a constant desire to improve the accuracy of the measurement system. In certain designs, the measurement system includes an autofocus system that is used to map the topography of the wafer surface along the optical axis prior to exposing the wafer. Subsequently, with information regarding the position along the optical axis, the wafer stage assembly can be controlled to properly position the wafer along the optical axis. Unfortunately, previous autofocus systems have been unsatisfactory, in that they can be slow, overly complex, and/or expensive, and/or such autofocus systems have only been able to achieve suboptimal results.